Microphones are properly used depending on purposes such as music, studio, conference, and video camera. Among these, for conference, a gooseneck condenser microphone is used in many cases because the direction of microphone can be changed freely and the microphone is not so conspicuous.
A typical conventional example is explained with reference to FIGS. 4 to 6. First, as shown in a general configuration view of FIG. 4, for a gooseneck condenser microphone, a microphone capsule 10 is used by being installed, for example, on a table, not shown, via a support pipe 20.
In this example, the support pipe 20 is provided with a base end portion 21 threadedly attached to a fixing member, not shown, on the table top side, a first flexible shaft 22, a telescopic intermediate pipe 23 whose length is adjustable, and a second flexible shaft 24 in that order from the lower side. At the upper end of the second flexible shaft 24, a capsule support section 25 is provided. All of these members are made of a metal. Since the flexible shafts 22 and 24 are flexible, the microphone capsule 10 can be directed to a suitable position with respect to a sound source (speaker).
Referring to FIG. 5, the microphone capsule 10 is provided with a cylindrical housing 11 made of, for example, aluminum, and although not shown, the housing 11 contains a condenser microphone unit in which a diaphragm stretchedly provided on a support ring and a backplate consisting of, for example, an electret board supported on an insulating seat are arranged opposedly via an electrical insulating spacer.
The back surface side of the housing 11 is closed by a back lid 12, and a contact pin 13 connected to the backplate protrudes from the back lid 12. Also, on the back surface side of the housing 11, a connection thread 14 having a internal thread 141 on the inside surface, which is for connection with the capsule support section 25, is fixed to the housing 11 so as to provide electrical conduction.
The capsule support section 25 is provided with a cylindrical housing 251 made of a brass material, for example, and in the housing 252, a circuit board 252 is arranged so as to close the interior of the housing 251. On the circuit board 252, an impedance converter (FET: Field Effect Transistor) 253 is mounted. On the lower surface side of the circuit board 252, one end of a microphone cord 30, which is pulled out by being inserted through the support pipe 20, is soldered.
To a gate terminal of the impedance converter 253, one side of a contact terminal 254 is connected by soldering, the contact terminal 254 consisting of a plate spring bent substantially into a V shape so as to be in elastic contact with the contact pin 13. The circuit board 252 is fixed in the housing 251 by a fixing ring 255.
Specifically, the fixing ring 255 has an external thread 255a on the outer peripheral surface thereof, and on the other hand, in the housing 251, a step portion 251a for receiving the circuit board 252 and an internal thread 251b threadedly engaged with the external thread 255a are provided. By threadedly engaging the fixing ring 255 with the internal thread 251b, the circuit board 252 is fixed so as to be urged against the step portion 251a. The screwing amount of the fixing ring 255 is about a lower half of the internal thread 251b, and an upper half thereof is left for connecting the microphone capsule 10.
The microphone capsule 10 and the capsule support section 25 are connected mechanically by threadedly engaging the internal thread 141 of the connection thread 14 with the upper half side of the external thread 255a. Accordingly, the contact pin 13 comes into elastic contact with the contact terminal 254, by which the microphone capsule 10 and the capsule support section 25 are also connected electrically.
As the microphone cord 30, a two-core shielded cable is used which includes a power line for supplying power to the condenser microphone unit in the microphone capsule 10, a signal line for sending an audio signal generated by the impedance converter (FET) to an output circuit section 40, described later, and a shield covered line for electrostatically shielding and grounding the power line and signal line.
FIG. 6 shows the lower surface side of the circuit board 252. At the periphery on the lower surface side of the circuit board 252, a gland pattern 252a is formed in a ring shape, and also at the periphery on the upper surface side of the circuit board 252, a gland pattern 252b is formed in a ring shape similarly. These gland patterns 252a and 252b are brought into conduction each other by plating in a through hole. Although not shown in FIG. 6, besides, the circuit board 252 is formed with electrode patterns of the gate, drain, and source of FET.
The shield covered line included in the microphone cord 30 is connected by soldering to the circuit board 252 so as to connect with the gland pattern 252a on the lower surface side. Also, the housing 11 of the microphone capsule 10 is electrically connected to the gland pattern 252b on the upper surface side via the connection thread 14 and the fixing ring 255, and the housing 251 of the capsule support section 25 is electrically connected to the gland pattern 252a on the lower surface side in a portion of the step portion 251a. 
Referring again to FIG. 1, the other end of the microphone cord 30 is pulled out of the base end portion 21 side of the support pipe 20, and is connected to the output circuit section (power module section) 40. The output circuit section 40 has a shield case 41. Although not shown, the shield case 41 contains an audio output circuit board including a low cut filter circuit and a transformer, a three-pin type output connector specified in EIAJ RC5236, and the like, and the output connector is connected with a phantom power source via a balanced shielded cable, not shown.
The microphone cord 30 consisting of the two-core shielded cable is vulnerable to (liable to be affected by) noise from the outside because the audio signal is transmitted imbalancedly. Therefore, if strong electromagnetic waves are applied to the microphone cord 30, the electromagnetic waves intrude into the condenser microphone unit in the microphone capsule 10 and the output circuit section 40, by which noise is sometimes generated.
In particular, in the gooseneck microphone constructed as described above, the housing 11 of the microphone capsule 10 and the support pipe 20 are commonly grounded in portions of the gland patterns 252a and 252b of the circuit board 252 arranged on the upper end side of the support pipe 20, so that the support pipe 20 sometimes acts as an antenna if it receives strong electromagnetic waves.
As a result, the electromagnetic waves received by the support pipe 20 intrude from the gland pattern to the grounding side of the impedance converter (FET) 253, and the impedance converter detects the waves, by which noise is generated.
In recent years, cellular phones have come into wide use. In the case where a cellular phone is used in the immediate vicinity of a microphone, the microphone receives considerably strong electromagnetic waves (for example, in the range of several centimeters to several tens centimeters, field intensity reaching several ten thousands of intensity of field generated in the city by commercial electric waves), so that measures against cellular phones are a pressing need in the field of microphone.
As one method for answering the need, a technique in which, for example, in a gun microphone in which the microphone unit is housed in a housing cylinder consisting of a conductor, the microphone unit is connected (grounded) to the housing cylinder consisting of a conductor at the shortest distance has been proposed in Patent Document 1 (Japanese Patent Application Publication No. 2001-103591).
However, although being effective for the gun microphone or the like, this method cannot be applied to a microphone in which, as in the conventional example, the microphone capsule and the audio output section are separated from each other and are connected to each other via the microphone cord.